Continuously variable torque range characteristics for a power transmission may be achieved using a friction belt and pulley arrangement. A drive pulley and a driven pulley, connected by an endless belt, are adapted to transfer torque across an infinitely variable torque range by adjusting the pitch diameter of the pulleys. The pitch diameter of the driving pulley increasing as the pitch diameter of the driven pulley decreases, and vice versa.
Continuously variable torque range characteristics may also be achieved using a hydraulic pump as a driving member and a hydraulic motor as a driven member. The pump and motor are located in a closed hydrostatic fluid pressure circuit. By varying the pump displacement, the effective speed ratio of the hydrostatic transmission can be changed through a wide torque range.
Various types of variable friction drives are also known. The relative positions of a driving cone member and a driven cone member may be adjusted to provide a variable torque range characteristic. Known friction cone drives typically have frictional contact between the surfaces of friction cones at a contact patch established between the cones; however, these systems may have a low level of torque capability. Friction torque at the contact patch is developed by a tangential force component on the surface of each cone member. Because of the geometry of the conical surfaces, the contact patch has incremental areas where a sliding motion occurs between the conical surfaces of the driving and driven members within the contact patch. This sliding motion requires the presence of a hydraulic lubricating oil film to avoid galling and deterioration of the friction surfaces of the conical members. The presence of an oil film, however, is imperfect protection against deterioration and wear of the friction surfaces.